Dyeing of cellulose-containing textile materials with vat dyes is generally known. In order to lend the required substantivity to the vat dye, which is insoluble in water, i.e. in order to fix it on the textile material, it is first necessary to convert it into its substantive water-soluble leuco form by reduction (vatting) and thereafter back into dyestuff pigment by oxidation.
In their leuco form, most vat dyes are distinguished by a high affinity to the fibers and therefore by high dye bath exhaustion of approximately 70 to 95%. In contrast thereto, leucoindigo is only absorbed by the fiber to approximately 10 to 20% in a single dyeing process. Because of this low dye bath exhaustion, dyeing with indigo by means of a discontinuous exhaustion process (dyeing from the bath) is problematical.
It is therefore customary to perform dyeing with indigo continuously "in several passages". This process only permits dyeing of yarn in the form of tows or as a warp sheet, but not dyeing of piece goods or yarn packages. With this process, the vatted indigo from several (mostly five or six) dye liquors of large volume with a low dyestuff concentration is applied by means of repeated, short (approximately 10 to 20 seconds) dye application by dipping the yarn through multiple dye vats and squeezing and oxidizing in the interposed air spaces between the dipping steps. However, dyeing in passages is considered to be only a stopgap measure, since it requires uneconomically large dyeing machinery and smaller dye batches cannot be realized. Furthermore, the repetition of the dyeing process results in extensive dyestuff penetration of the textile material so that, particularly with dark colors, extremely ring-dyed warp thread employed in jeans production, which is a prerequisite for generating the "stone-washed" effect in washing ofjeans, can only be obtained to an unsatisfactory degree.
A further problem when dyeing with indigo is that, because of the low dye bath exhaustion, the coloration obtained has only insufficient crocking fastness, since the dyestuff having remained in the gap volume is precipitated on the textile material after oxidation and is not fixed.
In the dyeing process described in prior German patent application 196 29 985.3, the substantivity of the leucoindigo is increased by performing dyeing at very high electrolyte concentrations (preferably approximately 280 g/l of dye liquor) and at pH values reduced (in comparison with conventional vat dyeing) to 10.2 to 11.3.